Decorating a customers kitchen I had a small area on the internal wall to patch that had blown due to external water penetration. Hacking a small section off I realised that a small part was dot dab (30x20cm) and the rest bonding. As this felt damp in certain areas I had it checked out by a damp company and was told it was purely left over moisture from the original water penetrating problem that had now been rectified and just needs hacking off to a metre and sand and cementing. This where I get confused reading what to do as previously I have only used bonding and a top coat.

The area in question is only around 1m sq and the deepest area that need to be built up is around 10cm, but only where the dot and dab area is. My many questions as a newbie are:

Do I need to use a mix of 3:1 sand cement or 4:1?
Do I need to add a waterproofer or anything else?
Do I use a sharp sand?
How long should and can I leave the sand cement undercoat before I finish with multifinish?
Do I need to PVA at any stage or do I just damp down prior to under and top coat?
Certain areas need building up (daubing out?) What is the max thickness I should apply the sand and cement and how long before I can apply the next coat to build up?

Do I need to use a mix of 3:1 sand cement or 4:1?
---4-1is fine
Do I need to add a waterproofer or anything else?
---Nope but make sure you do use a plasticiser though
Do I use a sharp sand?
---Nope use normal building sand.
How long should and can I leave the sand cement undercoat before I finish with multifinish?
---A couple of days at least to dry before you skim.
Do I need to PVA at any stage or do I just damp down prior to under and top coat?
---- Just damp the wall down prior to scratch coat.
Certain areas need building up (daubing out?) What is the max thickness I should apply the sand and cement and how long before I can apply the next coat to build up?
----You can keep building out the same day as long as wall still has some suction to it if not have to be the next day and the next until you get a level finish.

you should control the suction with something yes otherwise the finish could dry out too quickly and blow or crack, PVA or WBA or Bond-it, the latter 2 more expensive and probably unnecessary on this occasion.

broadbanit, i've always been told to use washed plastering sand for rendering to avoid any efflorescence, have i been told a porky pie?

you should control the suction with something yes otherwise the finish could dry out too quickly and blow or crack, PVA or WBA or Bond-it, the latter 2 more expensive and probably unnecessary on this occasion.

broadbanit, i've always been told to use washed plastering sand for rendering to avoid any efflorescence, have i been told a porky pie?

Its a strange one tbh i noticed someone was pushing to use washed plastering sand the other week in a post. I personally have never used it have always used what i would call red sand with a 4-1 mix and never had any problems. Just completed in January a new build 4 bed house all in cement render and board finish skim was such fun to get back to doing it the traditional way as the architect said lol (but my right arm did ache a fair bit)
Maybe i am just to old school i always used washed sand for floor screed but now only ever use ready mix for floors over 10sm.

Nice one thats very very helpful indeed. So regards my scratch coat and leaving for a few days at least, will I need to damp down or PVA prior to top coat?

Cheers

If you leave it for a few days damping down would be fine if only going to be a small area. A large cement rendered wall say 6-8 mtrs you can PVA it (to hold the suction back) but be prepared for the thistle skim to take a while to dry.(good option if you are not used to skimming over sand & cement). Otherwise just go for it just don't be tempted to throw a couple of walls on like you would on dry-lined walls as that would make you break out in a bit of sweat .

Nice one chaps, all proving very useful as tomorrow I will be starting the work. This plasterers washed sand, is this what I need to look for or is it called something else in my local selco or wickes store?

Regards plasterciser I also need to add, I'm presuming there is a standard ratio to mix in?

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